1. Field of the Invention
The invention relates to a process for thickening a fibrous pulp suspension by guiding the suspension between a rigid first face and a flexible second face that move in relation to each other and form a converging draining gap where at least one of the first face and the second face is water permeable.
2. Discussion of Background Information
Processes of this type are used either to increase the pulp density of a fibrous pulp suspension, or to remove undesirable components from the fibrous pulp suspension by washing. These processes are associated with each other because draining naturally is necessary to wash a fibrous pulp suspension. Washing processes generally start with relatively low initial pulp densities (such as 1-3%). However, the effectiveness of a washing process is a function of other parameters as well, and not only of the increase in pulp density.
Such processes can be carried out with various known devices for thickening a fibrous pulp suspension. These include drum thickeners, a wide variety of screen (sieve) presses, and filter presses.
In a screen press, the fibrous pulp suspension is introduced and pressed between a screen and a roll, or between two screens, so that water is expelled. The suspension may be introduced either with a material container (a chest) that provides the suspension in the region of the screen or screens, or by a head box which forms a free jet of suspension material. For technical reasons, such as sealing problems, a material container cannot be used when suspension pulp densities are too low.
Drum thickeners form a fibrous web on the outside surface of a drum that dips into the suspension, and this fibrous web is drained into the interior of the drum. The suspension can have a relatively low pulp density. Due to the circumferential movement of the drum, the fibrous web is transported out of the suspension container, and can be removed as a thickened pulp. It then often is an acceptable material for further processing. Other procedures have been carried out for further increasing the pulp density of the pulp that has come out of the suspension and adheres to the drum. In particular, couch rolls may be used, which impart additional pressure on the fibrous web. On the other hand, a revolving belt may be pressed against the web, as disclosed in U.S. Pat. No. 2,278,525 or in WO 94/08088. As a general rule, such measures do achieve further increases in pulp densities, but the increases often are slight, and these pulp density increases may be reduced in later processing, for example by remoistening as a fibrous pulp belt emerges from a nip.
In the drum thickener disclosed in U.S. Pat. No. 1,421,364, a vat is completely filled with suspension during operation. The polygonal outer surface of the drum conveys the thickened pulp out along arched sealing faces.
WO 96/08600 describes another draining device. It likewise contains a rotating drum with a permeable surface. On a portion of its circumference, the drum is enveloped by an impermeable plastic belt. A suspension is sprayed with the aid of a head box into the gap formed between the drum and the plastic belt. A vacuum is applied from the inside of the drum in the region where the pulp jet reaches the drum wall. Such a machine is very expensive to manufacture and operate.
U.S. Pat. No. 4,750,340 discloses another device suitable for pulp thickening, with a rotary driven, perforated drum that is surrounded by an arched plate which is fixed to the drum in a slightly eccentric manner. As a result, a nip is formed between the drum and the wall, into which an already pre-thickened pulp is introduced by way of a feed worm drive. Such a device also is very costly and it is particularly suited for higher initial pulp densities.